Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Touch screen input devices or sensors for computers and other consumer electronics devices such as mobile phones, personal digital assistants (PDAs) and hand-held games are highly desirable due to their extreme ease of use. In the past, a variety of approaches have been used to provide touch screen input devices. The most common approach uses a flexible resistive overlay, although the overlay is easily damaged, can cause glare problems, and tends to dim the underlying screen, requiring excess power usage to compensate for such dimming. Resistive devices can also be sensitive to humidity, and the cost and drive power consumption of the resistive overlay scale quadratically with perimeter. Another approach is the capacitive touch screen, which also requires an overlay. In this case the overlay is generally more durable, but the glare and dimming problems remain.
In yet another common approach, a matrix of light beams (usually infrared) is established in front of a display with a touch event detected by the interruption of one or more of the beams. Such “optical” touch screens have long been known (U.S. Pat. Nos. 3,478,220; 3,673,327), with the beams generated by arrays of optical sources such as light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) and detected by corresponding arrays of detectors (such as phototransistors or photodiodes). This type of optical touch screen has the advantage of being overlay-free and can function in a variety of ambient light conditions (U.S. Pat. No. 4,988,983), but has a major cost problem in that it requires a large number of source and detector components, as well as supporting electronics. Since the spatial resolution of such a system depends on the number of sources and detectors, this component cost increases with resolution.
U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. Patent Application Nos. 2002/0088930 A1 and 2004/0201579 A1, each of which is incorporated herein by reference in its entirety, disclose an improved type of optical touch screen sensor, where waveguides are used to distribute and collect the matrix of light beams. As discussed below with reference to FIG. 1, this approach requires only a single optical source and a single multi-element detector representing a substantial cost reduction.
FIG. 1 illustrates the operation of an optical touch screen sensor 10 similar to that described in U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. patent application Ser. Nos. 2002/0088930 A1 and 2004/0201579 A1. In this optical touch screen sensor design a single optical source 11 (such as an LED or a VCSEL) launches light into an array of “transmit” integrated optical waveguides 12, via some form of 1.times.N splitter 13. Optical waveguides 12 then launch an array of parallel, spaced apart light beams 14 across a display area 15, which are then collected at the other side of the display area by a similar array of “receive” integrated optical waveguides 16 and conducted to a position-sensitive (i.e. multi-element) detector 17. A touch event 18 (e.g. by a finger or stylus) is detected as a shadow 19, with position determined from the particular beam(s) blocked by the touching object.
The touch screen sensors are usually two dimensional and rectangular, with two arrays (X, Y) of transmit waveguides along adjacent sides of the display area, and two corresponding arrays of receive waveguides along the other two sides of the display area. In one construction, a single optical source launches light into a plurality of waveguides that form both the X and Y transmit arrays. In another version, separate optical sources are used for each of the X and Y transmit arrays. On the transmit side, the waveguide arrays may guide light from the optical source to rows of lens elements 110 that expand the guided light beams in the horizontal (i.e. X, Y) plane, then collimate them in the horizontal plane as they are launched across the display area. Collimation in the vertical plane may be achieved with an external vertical collimating lens (VCL), for example a cylindrical lens (not shown in FIG. 1). The receive side is essentially identical, and on each side the arrays of waveguides and lens elements are positioned within and protected by the bezel of the screen. To minimize the width of the bezel, it is desirable for the transmit and receive elements to be as short as possible.
As is usual with integrated optical waveguides, “transmit” optical waveguides 12 and “receive” optical waveguides 16 each consist of an array of patterned, light guiding cores (of refractive index n1) surrounded by a cladding (of refractive index n2, where n2<n1) and mounted on a mechanically robust substrate. Frequently, the portion of cladding between the light guiding cores and the substrate is referred to as the “lower cladding” or “bottom cladding”, with the remainder of the cladding referred to as the “upper cladding” or “top cladding”.
In the type of optical touch screen sensor described above, each “receive” waveguide 16 is in optical communication with an integral number (usually one, but possibly more) of individual elements 111 of multi-element detector 17. It will be appreciated that for this system to accurately determine the position of a touch event, it is crucial that the light in each of the spaced apart beams 14 be faithfully guided by receive waveguides 16 to the respective elements 111 of multi-element detector 17. If the individual elements 111 of multi-element detector 17 receive light from any source other than the appropriate receive waveguides 16, the performance of the touch screen sensor, as measured for example by the signal-to-noise ratio at the detector, will be degraded. One possible source of stray light is ambient light captured by the transmit- or receive-side lenses. Another possible source of stray light is signal light in spaced apart beams 14 that gets captured by the substrate or cladding of receive waveguides 16, instead of by the waveguide cores.
The present disclosure ameliorates or at least provides a commercial alternative to the prior art and that, at least in the preferred embodiments improves performance of optical touch screen sensors.